Student Exploration: Penumbra Effect

Student Exploration: Penumbra Effect Vocabulary: eclipse, penumbra, point source, umbra Prior Knowledge Questions (Do these BEFORE using the Gizmo.) Try this experiment: Shine a flashlight (or a desk lamp) on your hand so that it casts a shadow on a sheet of white paper. Hold your hand about 15-30 cm (6-12 inches) above the paper. 1. Look at the edges of the shadow. Are they sharp or fuzzy? Soft. Not fuzzy, but not sharp. 2. Move your hand up and down. How does this affect the fuzziness of the shadow? The closer I move my hand to the flashlight, the fuzzier it gets. 3. Why do you think the shadow has fuzzy edges? I think it has fuzzy edges because it is getting o es because it is getting closer, like how things start to get blurry when you look too close. Gizmo Warm-up The fuzzy edge of a shadow is the penumbra, a region where light is only partially blocked. The Penumbra Effect Gizmo demonstrates why penumbras form. To begin, check that the Number of lights is 1. Click the light to turn it on. The gray block casts a shadow. 1. The light is a point source, which means that all of the light rays emanate from a single point. Does the resulting shadow have sharp or fuzzy edges? sharp 2. Increase the Number of lights to 2. How does this affect the shadow? The darker shadow is sharp, the lighter shadow is fuzzy. 3. The central, dark part of the shadow is the umbra. Would you be able to see either of the lights if you were standing in the umbra? Explain. No, you'd only see the block that's blocking the light, in the case that only your head is in the umbra. If you were just standing normally with your abdomen being in the umbra, you might see the light on top. Activity: Light gradations Get the Gizmo ready: Check that the Number of lights is 2. Be sure the Light intensity on front of detector checkbox is turned on. Question: What causes a penumbra to form? 1. Observe: The white rectangle behind the gray block is a detector that measures the intensity of light on its surface. Move the detector into the top part of the penumbra. A. If you were standing where this detector was located, which light(s) would you be able to see? the top one B. Turn off the top light. Is the detector now in the light or in the dark? the dark C. Turn on the top light and turn off the bottom light. Is the detector in the light or the dark now? Not in umbra.or penumbra. It's just there. D. Turn on the bottom light and Show ray trace. How does the ray trace of each light help you determine which light can be seen from the detector? The ray trace shows where the light is blocked 2. Summarize: Based on what you have observed so far, what causes a penumbra to form? When some light is blocked and there is another light, a second (lighter) shadow will form (penumbra). 3. Predict: In the two-light example, there were three possible light intensity values on the detector: two lights, one light, or no lights. How many possible light intensity values do you expect if there were four lights? 4 4. Test: Set the Number of lights to 4. A. Describe the resulting penumbra. there are four different intensities, & total segments, one of the two intensities on each side (top or bottom). B. How many different light intensity values result from four light sources? 4 (Activity continued on next page) Activity (continued from previous page) 5. Apply: The diagram below shows the situation with five lights. The lights are labeled A-E. In each part of the penumbra, list the lights that are visible from that location. Use the Gizmo for help as you do this. abcd ABCDE abcde abc ab a e abcde -de cde bcde 6. Explore: Change the Number of lights to 2, the Light spacing to 8, the Shadow block width to 5, and the Distance to light(s) to 25. Then use the Gizmo to explore each of the following changes. A. How does increasing the Light spacing change the shape of the umbra and penumbra? It spreads out the penumbra and compresses the umbra B. How does increasing the Shadow block width change the shape of the umbra and penumbra? It makes the umbra wider and keeps the penumbra the same width. C. How does increasing the Distance to light(s) change the shape of the umbra and penumbra? It makes the umbra wider and penumbra smaller 7. Make a connection: During an eclipse, one body moves into the shadow of another. What do you think you would see during a lunar eclipse, when the Moon moves into Earth's shadow? (Hint: The Sun is a wide source of light, not a point source.) You wouldn't see anything because the shadow of earth would block out the suns light. The moon does not produce it's own light, it only reflects off the sun. But since the earth is blocking the sun's light during a lunar eclipse, you wouldn't see anything.